docs: def children(self):
          """~TermList: The direct children of the `Term`.
          """
          if self._children is None:
              topdowns = tuple(Relationship.topdown())
              self._children = TermList()
              self._children.extend(
                  [ other
                      for rship,others in six.iteritems(self.relations)
                          for other in others
                              if rship in topdowns
                  ]
              )
          return self._children